Problem: Robin can clean $72$ rooms in $6$ days. How many rooms can Robin clean in $9$ days?
Answer: We're trying to find the number of rooms Robin can clean in $9$ days. If in ${6}$ days, Robin can clean ${72}$ rooms, then in $1$ day, she can clean $12$ rooms.  So, in ${9}$ days, Robin can clean ${108}$ rooms.